1. Field of the Invention
The present invention relates to a planetary differential gear type reduction device for reducing the rotation speed of a drive source to one necessary for a desired object, a driving device with a reduction mechanism using such a reduction device, and an image forming apparatus using the driving device.
2. Description of the Background Art
It is a common practice with an image forming apparatus to transfer the output torque of a motor or similar drive source to a desired rotary body via speed reducing means. For this purpose, use is generally made of a gear train, a worm gear or a belt reduction mechanism. The rotary body is any one of a photoconductive drum, an image transfer belt, a fixing roller and so forth. While the rotary body rotates at a speed of several ten to several hundred revolutions per minute, the motor or drive source generates rotates at a speed of several thousand revolutions per minute. Therefore, to cause the rotary body to rotate at the above, speed, speed reducing means with a reduction ratio of about 1/10 to 1/30 is necessary. The accuracy of rotation of such a rotary body has critical influence on the quality of images. However, it is difficult with the conventional speed reducing means to accurately transmit a constant-speed torque from the drive source to the rotary body due to limited machining accuracy and limited assembling accuracy.
Some different methods are available for realizing the accurate transmission of a constant-speed torque. For example, a pitch irregularity frequency band in which the variation of speed transmission (variation of rotation speed) is conspicuous may be raised, i.e., a perceivable spatial frequency (cycle/mm) may be raised. With this method, it is possible to reduce the banding, jitter and granularity of an image particular to an image forming apparatus. In this case, the pitch irregularity of an image should preferably lie in a band ranging from the order of several centimeters to several ten micrometers to the order of several micrometers. Another method for the accurate transmission of a constant-speed torque is reducing the number of reduction steps, i.e., the number of locations where the rotation speed varies. Other methods include using a reduction mechanism resistive to load variations to thereby reduce backlash ascribable to the accumulation of the number of reduction steps, guaranteeing a sufficient inertia moment, and preventing gear accuracy from falling due to thermal expansion ascribable to temperature elevation inside the apparatus.
Reducing the number of reduction steps is particularly is effective to realize the accurate transmission of a constant-speed torque. Further, a decrease in the number of reduction steps reduces the number of structural parts and therefore cost, obviates the critical fall of transmission efficiency, and reduces noise ascribable to the intermeshing of gears. In addition, such a method implements a small size, compact image forming apparatus.
A large reduction ratio is required in fields other than the field of image forming apparatuses, too. Various methods have been proposed in the past for implementing a large reduction ratio and include one using a planetary gear mechanism, one using an internal gear mechanism with a small difference in the number of teeth, and one using a harmonic drive mechanism. The problem with the planetary gear mechanism is that a reduction ratio is generally limited to about 1/10 for various reasons. A reduction ratio above 1/10, e.g., a reduction ratio of 1/20 is not achievable without resorting to two reduction steps, which complicate the construction and need an extra space. The internal gear mechanism is not practicable without resorting to a balance weight because planetary gears rotate at high speed with eccentricity. Although the harmonic drive mechanism involves no backlash and therefore achieves high positioning accuracy, it constantly needs preload with the result that a heavy load acts even during idling.
When any one of the reduction schemes described above is applied to, e.g., a photoconductive drum, use is generally made of an internal gear. However, when an internal gear and a spur gear (pinion) are intermeshed for speed reduction, a desired reduction ratio cannot be easily selected due to various limitations including involute interference, trochoid interference and trimming.
In light of the above, there should preferably be constructed speed reducing means capable of accurately transmitting a constant-speed rotation and implementing a great reduction ratio without using an internal gear and with a minimum of reduction steps. This kind of speed reducing means may be implemented as a planetary differential gear type reduction device taught in, e.g., “HANDBOOK OF EXAMPLES OF MODERN MACHINE TECHNOLOGIES”, GENDAI KOGAKU-SHA, Volume 2, page 679, section 2717. The planetary differential gear type reduction device includes four-joint planetary gears including two external gears. More specifically, two planetary gears whose numbers of teeth are Z2 and Z4, respectively, are coaxially mounted on the ends of an arm affixed to an input shaft. A stationary gear having a number of teeth of Z3 and an output shaft mounted on an output shaft and having a number of teeth of Z5 are held in mesh with the two planetary gears. However, because the two planetary gears are different in the number of teeth, the input shaft and output shaft cannot be made coaxial in a single module unless the numbers of teeth mentioned above satisfy a relation:Z2+Z3=Z4+Z5  Eq.(1)
A reduction ratio available with this type of reduction device is expressed as:N5=N1×(Z2×Z5−Z3×Z4)/(Z2×Z5)  Eq. (2)where N1 denotes the rotation speed of the input shaft, and N5 denotes the rotation speed of the output shaft.
To implement a reduction ratio of about 1/10 with this type of reduction device, Z2 and Z5 must be, e.g., twenty each while Z3 and Z4 must be nineteen each. Further, for a reduction ratio of about 1/20, Z2 and Z5 must be, e.g., forty each while Z3 and Z4 must be thirty-nine each. Moreover, for a reduction ratio of about 1/30, Z2 and Z5 must be, e.g., sixty each while z3 and Z4 must be fifty-nine each. In this manner, the number of teeth required of the individual gear increases with an increase in reduction ratio, resulting in a bulky gear mechanism. This increases the diameter of revolution of the planetary gears mounted on the arm. This type of reduction gear is therefore not feasible for high-speed rotation and makes the apparatus large size.
Technologies relating to the present invention are also disclosed in, e.g., Japanese Utility Model Publication No. 6-20935 and Japanese Patent Laid-Open Publication Nos. 5-126212, 8-334164 and 2001-173733.